This invention relates to electrophoretic displays, and methods for operating such displays. More specifically, this invention relates to electrophoretic displays in which solid particles and a suspending fluid are held within a cavity.
Electrophoretic displays have been the subject of intense research and development for a number of years. Such displays use a display medium comprising a plurality of electrically charged particles suspended in a fluid. Electrodes are provided adjacent the display medium so that the charged particles can be moved through the fluid by applying an electric field to the medium. In one type of such electrophoretic display, the medium comprises a single type of particle having one optical characteristic in a fluid which has a different optical characteristic. In a second type of such electrophoretic display, the medium contains two different types of particles differing in at least one optical characteristic and in electrophoretic mobility; the particles may or may not bear charges of opposite polarity. The optical characteristic which is varied is typically color visible to the human eye, but may, alternatively or in addition, be any one of more of reflectivity, retroreflectivity, luminescence, fluorescence, phosphorescence, or (in the case of displays intended for machine reading) color in the broader sense of meaning a difference in absorption or reflectance at non-visible wavelengths.
Electrophoretic displays can be divided into two main types, namely unencapsulated and encapsulated displays. In an unencapsulated electrophoretic display, the electrophoretic medium is present as a bulk liquid, typically in the form of a flat film of the liquid present between two parallel, spaced electrodes. Such unencapsulated displays typically have problems with their long-term image quality which have prevented their widespread usage. For example, particles that make up such electrophoretic displays tend to cluster and settle, resulting in inadequate service-life for these displays.
An encapsulated, electrophoretic display differs from an unencapsulated display in that the particle-containing fluid is not present as a bulk liquid but instead is confined within the walls of a large number of small capsules. Encapsulated displays typically do not suffer from the clustering and settling failure mode of traditional electrophoretic devices and provides further advantages, such as the ability to print or coat the display on a wide variety of flexible and rigid substrates.
For further details regarding encapsulated electrophoretic displays, the reader is referred to U.S. Pat. Nos. 5,930,026; 5,961,804; 6,017,584; 6,067,185; 6,118,426; 6,120,588; 6,120,839; 6,124,851; 6,130,773; 6,130,774; 6,172,798; 6,177,921; 6,232,950; 6,249,271; 6,252,564; 6,262,706; 6,262,833; 6,300,932; 6,312,304; 6,312,971; 6,323,989; 6,327,072; 6,376,828; 6,377,387; 6,392,785; 6,392,786; 6,413,790; 6,422,687; 6,445,374; 6,445,489; 6,459,418; 6,473,072; 6,480,182; 6,498,114; 6,504,524; 6,506,438; 6,512,354; 6,515,649; 6,518,949; 6,521,489; 6,531,997; 6,535,197; 6,538,801; 6,545,291; and 6,580,545; and U.S. Patent Applications Publication Nos. 2002/0019081; 2002/0021270; 2002/0053900; 2002/0060321; 2002/0063661; 2002/0063677; 2002/0090980; 2002/0106847; 2002/0113770; 2002/0130832; 2002/0131147; 2002/0145792; 2002/0171910; 2002/0180687; 2002/0180688; 2002/0185378; 2003/0011560; 2003/0011867; 2003/0011868; 2003/0020844; 2003/0025855; 2003/0034949; 2003/0038755; 2003/0053189; 2003/0076573; 2003/0096113; 2003/0102858; 2003/0132908; 2003/0137521; 2003/0137717; and 2003/0151702; and International Applications Publication Nos. WO 99/67678; WO 00/05704; WO 00/38000; WO 00/38001; WO 00/36560; WO 00/67110; WO 00/67327; WO 01/07961; and WO 01/08241. All of these patents and applications are in the name of, or assigned to, the Massachusetts Institute of Technology (MIT) or E Ink Corporation.
Many of the aforementioned patents and applications recognize that the walls surrounding the discrete microcapsules in an encapsulated electrophoretic medium could be replaced by a continuous phase, thus producing a so-called “polymer-dispersed” electrophoretic display in which the electrophoretic medium comprises a plurality of discrete droplets of an electrophoretic fluid and a continuous phase of a polymeric material, and that the discrete droplets of electrophoretic fluid within such a polymer-dispersed electrophoretic display may be regarded as capsules or microcapsules even though no discrete capsule membrane is associated with each individual droplet; see for example, U.S. Pat. No. 6,392,786, at column 6, lines 44-63. See also the aforementioned U.S. Patent Application Publication No. 2002/0131147, and the corresponding International Application PCT/US02/06393 (Publication No. WO 02/075443). Accordingly, for purposes of the present application, such polymer-dispersed electrophoretic media are regarded as sub-species of encapsulated electrophoretic media.
A related type of electrophoretic display is a so-called “microcell electrophoretic display”, sometimes also called a “microcup” electrophoretic display. In a microcell electrophoretic display, the charged particles and the suspending fluid are not encapsulated within microcapsules but instead are retained within a plurality of cavities formed within a carrier medium (or substrate), typically a polymeric film. See, for example, International Applications Publication No. WO 02/01281, and published US Application No. 2002/0075556, both assigned to Sipix Imaging, Inc.
Hereinafter, the term “microcavity electrophoretic display” will be used to cover both encapsulated and microcell electrophoretic displays.
One of the problems with electrophoretic displays is the limited range of colors which each pixel of the display can produce. The prior art describes two main types of electrophoretic media. One type comprises a single type of electrically charged particle in a colored medium. This type of medium is only capable of producing two colors at each pixel; either the color of the particles or the color of the medium is seen. The second type of medium comprises two different types of electrically charged particles in an essentially uncolored medium; the two types of particles may differ in polarity of electric charge or have charges of the same polarity but differ in electrophoretic mobility. Again, this type of medium is only capable of producing two colors at each pixel, namely the colors of the two types of particles.
One approach to expanding the limited range of colors available from conventional electrophoretic displays is to place an array of colored filters over the pixels of the display. For example, consider the effect on a display comprising white particles in a black fluid of placing an array of color filters (say red, green and blue) over the individual pixels of the display. Moving the white particles adjacent the viewing surface of a pixel covered with a red filter would color that pixel red, whereas moving the white particles of the same pixel adjacent the rear surface of the display would render the pixel black. The main problem with this approach to generating color is that the brightness of the display is limited by the pixilation of the color filter. For example, if a red color is desired, the pixels covered by red filters are set to appear red, whereas the pixels covered by green and blue filters are set to appear dark, so that only a fraction of the display surface has the desired color while the remaining portion is dark, thus limiting the brightness of any color obtained. A reflective display that was capable of three optical states (black, white and color or black, white and transparent) would significant advantages in image quality, cost and ease of manufacture.
Conventional electrophoretic displays rely upon movement of electrically charged particles in an electric field under electrostatic forces; the particles move along the lines of force of the electric field. However, it is known that objects can be moved by dielectrophoretic forces, that is to say that dipoles induced in the objects by a non-uniform electric field cause the particles to move towards regions of higher field strength. See, for example, U.S. Pat. No. 4,418,346 to Batchelder which describes an apparatus for providing a dielectrophoretic display of visual information. In this apparatus, a “bubble” of a fluid is moved through a second, immiscible fluid in a stepwise manner by applying voltages to closely spaced electrodes, the bubble being visible against a visually contrasting background. Visual information is conveyed by the position of the bubble relative to the background. The patent suggests that a simple one-dimensional display of this type might represent the level of an analog signal by the position of the bubble. However, since the movement involved is that of a bubble against a contrasting background, such an apparatus does not appear capable of displaying an arbitrary image.
It has now been realized that using a microcavity electrophoretic medium in a dielectrophoretic display greatly simplifies the problem of generating the heterogeneous electric field required by such a display, since differences between the dielectric constant and/or conductivity between the suspending fluid and the material surrounding the suspending fluid (such as a polymeric binder in which the capsules are embedded, as described in many of the aforementioned E Ink and MIT patents and publications, or the substrate in which the cavities of a microcell electrophoretic display are formed) will result in a heterogeneous electric field which can be used to move the particles within the suspending fluid against the side walls of the cavities, thereby providing the display with a substantially transparent state.
It has further been realized that a microcavity electrophoretic medium allows for the provision of auxiliary electrodes which can be used to assist the movement of particles required to provide the display with a substantially transparent state.
Variable transmission windows represent a potentially important market for electrophoretic media. As the energy performance of buildings and vehicles becomes increasingly important, electrophoretic media can be used as coatings on windows (including skylights and sunroofs) to enable the proportion of incident radiation transmitted through the windows to be electronically controlled by varying the optical state of the media. Effective implementation of such “variable-transmissivity” (“VT”) technology in buildings is expected to provide (1) reduction of unwanted heating effects during hot weather, thus reducing the amount of energy needed for cooling, the size of air conditioning plants, and peak electricity demand; (2) increased use of natural daylight, thus reducing energy used for lighting and peak electricity demand; and (3) increased occupant comfort by increasing both thermal and visual comfort. Even greater benefits would be expected to accrue in an automobile, where the ratio of glazed surface to enclosed volume is significantly larger than in a typical building. Specifically, effective implementation of VT technology in automobiles is expected to provide not only the aforementioned benefits but also (1) increased motoring safety, (2) reduced glare, (3) enhanced mirror performance (by using an electro-optic coating on the mirror), and (4) increased ability to use heads-up displays. Other potential applications of VT technology include privacy glass and glare-guards in electronic devices.
In addition to the aforementioned advantages, variable transmission windows can be used to provide privacy. Conventional (invariable) glass windows, doors, and room partitions have the advantage of admitting natural light, but the disadvantage of not providing privacy when needed. Mechanical privacy devices such as shades, for example in the form of vertical blinds, take up space, are cumbersome to use, and are often unreliable. Privacy “veiling” can be achieved by increasing the reflection, absorption, and scattering of the glass, using mirrored, tinted or frosted glass respectively. The veiling effect increases with the ratio of the light that is reflected, absorbed, or scattered by the window to the light that is transmitted through the window. Only scatter screening provides a veiling effect that is independent on the illumination conditions in- and outside the window because scatter screening blurs the view through the window, whereas reflection or absorption veiling becomes ineffective when the illumination beyond the window is high compared to that surrounding an observer outside the window.
Frosted glass in doors, room partitions, and windows is frequently used not only to provide privacy but also to display information e.g. the name and opening hours of a business that occupies the premises whose privacy is protected by the frosted glass. Frosted glass is also frequently used for decorative purposes i.e. displaying decorative patterns. However, conventional frosted glass has two major disadvantages; the privacy screening cannot be switched off when not needed, and information on the glass cannot be changed easily and without high costs when necessary, for example when the tenant of a building changes.
Liquid crystal (LC) display technology can be used instead of frosted glass for ‘variable opacity’ privacy windows, and can also display information. However, LCD windows of any useful size are very costly, and high in power consumption.
Accordingly, variable transmission-windows using electrophoretic media have been proposed; see for example U.S. Pat. Nos. 7,327,511; 7,999,787; and 7,679,814. Such electrophoretic VT windows have the advantages of low cost per area, and require power only when switching, and offer potential for in situ deposition by spray coating. However, such windows can suffer from visible non-uniformities (mottle, streaks, or point defects)) that may make them less attractive than liquid crystal-based windows despite their reduced cost and power consumption. Achieving uniformity below visible threshold of non-uniformity in electrophoretic VT windows may erase some of the cost benefits of such windows over their liquid crystal based counterparts.
The present invention provides a variable transmission electrophoretic window (or other display) which renders non-uniformities in the electrophoretic medium less visible.